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The endoplasmic reticulum (ER) plays an important role in protein biosynthesis. Proteins that are synthesized by ribosomes on the ER are transported to the Golgi apparatus for processing. Some of these proteins will be secreted from the cell, others will be inserted into the plasma membrane, and still others will be inserted into lysosomes.

Protein Synthesis Is The Primary Function Of

The endoplasmic reticulum (ER) is a system of membranes (flattened vesicles) that extend throughout the cytoplasm. Often it is more than half of the total membrane in the cell. This structure was first noted in the late 19th century, when studies of stained cells revealed the presence of some kind of extensive cytoplasmic structure, then called gastroplasm. Electron microscopy made it possible to study the morphology of this organism in the 1940s, when it was given its current name.

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Endoplasmic reticulum can be divided into two functionally distinct forms, smooth endoplasmic reticulum (SER) and rough endoplasmic reticulum (RER). The morphological difference between the two is the presence of protein synthesis particles, called ribosomes, attached to the outer surface of the RER.

The functions of the SER, a meshwork of fine tubular membrane vesicles, vary considerably from cell to cell. An important role is the formation of phospholipids and cholesterol, which are important components of plasma and internal membranes. Phospholipids are composed of fatty acids, glycerol phosphate, and other small water-soluble molecules that are attached to the ER membrane by enzymes whose active sites are exposed to the cytosol. Some phospholipids reside in the ER membrane, where, with the help of specific enzymes within the membrane, they can “flip” from the cytoplasmic side of the bilayer, where they are formed, to the exoplasmic, or internal, side. This process ensures symmetrical expansion of the ER membrane. Other phospholipids are transported through the cytoplasm to other membrane structures, such as the cell membrane and the mitochondrion, by specialized phospholipid transfer proteins.

In liver cells, SER is specialized for the detoxification of various compounds produced by metabolic processes. Liver SER contains several enzymes called cytochrome P450, which catalyze the breakdown of carcinogens and other organic molecules. In the cells of the adrenal glands and gonads, cholesterol is converted to SER in one step of its conversion to steroid hormones. Finally, the SER in muscle cells, known as the sarcoplasmic reticulum, sequesters calcium ions from the cytoplasm. When a muscle is stimulated by a nerve, calcium ions are released, causing the muscle to contract.

The RER is usually a series of interconnected flattened sacs. It plays a central role in the synthesis and export of proteins and glycoproteins and these functions are particularly well studied in secretory cells. Among the many secretory cells in the human body are liver cells secreting serum proteins such as albumin, endocrine cells secreting peptide hormones such as insulin, salivary gland and pancreatic acinar cells secreting digestive enzymes, mammary gland cells secreting milk proteins, and cartilage cells secreting collagen. and secreting cartilage cells. proteoglycans.

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Ribosomes are the particles that synthesize proteins from amino acids. They consist of four RNA molecules and between 40 and 80 proteins are assembled into a large and a small subunit. Ribosomes are either free (ie, not membrane-bound) in the cytoplasm of the cell or bound to the RER. Lysosomal enzymes, proteins destined for the ER, Golgi, and cell membranes, and proteins secreted from the cell are among those synthesized on membrane-bound ribosomes. Free ribosomes are proteins that remain in the cytosol and are attached to the inner surface of the outer membrane, as well as incorporated into the nucleus, mitochondria, chloroplasts, peroxisomes, and other organelles. Special features of proteins label them for transport to specific destinations inside or outside the cell. In 1971, German-born cellular and molecular biologist Gunter Bühl and Argentine-born cellular biologist David Sabatini proposed that the amino-terminal part of a protein (the first part of the molecule to be made) could act as a “signal sequence”. They proposed that such a signal sequence facilitates binding of the growing protein to the ER membrane and brings the protein either into the membrane or through the membrane into the ER lumen (internalization).

The signaling hypothesis has been substantiated by a large body of empirical evidence. Translation of the blueprint for a specific protein encoded in a messenger RNA molecule begins on a free ribosome. As the growing protein, with the signal sequence at its amino-terminal end, leaves the ribosome, the sequence binds to a complex of six proteins and an RNA molecule called the signal recognition particle (SRP). SRP also binds to ribosomes to prevent further protein synthesis. The ER membrane contains receptor sites that bind the SRP-ribosome complex to the RER membrane. Upon binding, translation resumes, the SRP dissociates from the complex and the signal sequence and nascent protein cascade through the rest of the membrane, through a channel called the translocon, into the ER lumen. At that point, the protein is permanently separated from the cytosol. In most cases, the signal sequence is cleaved from the protein by an enzyme called signal peptidase as it unfolds on the luminal surface of the ER membrane. In addition, in a process known as glycosylation, oligosaccharide (complex sugar) chains are added to most proteins to form glycoproteins. Within the ER lumen, proteins are folded into their characteristic three-dimensional structure.

Within the lumen, proteins are secreted from the cell in the transitional region of the ER, a region that is largely free of ribosomes. There the molecules are packaged into small membrane-bound transport vesicles, which detach from the ER membrane and move through the cytoplasm to the target membrane, usually the Golgi complex. There the transport vesicle membrane fuses with the Golgi membrane, and the contents of the vesicle are delivered into the lumen of the Golgi. This, like all the processes of vesicle budding and fusion, preserves the side of the membrane; That is, the cytoplasmic surface of the membrane always faces the outside, and the luminal contents are always separated from the cytoplasm.

Some non-secretory proteins formed on the RER are part of the cell membrane system. In these membrane proteins, in addition to the signal sequence, one or more anchor regions contain lipid-soluble amino acids. The amino acid prevents the protein from fully entering the ER lumen by anchoring it to the phospholipid bilayer of the ER membrane.

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The Golgi complex is the site of modification, synthesis, and export of secretory proteins and glycoproteins. This organelle, first described in 1898 by the Italian cytologist Camillo Golgi, is a specialized structure consisting of five to eight flattened, disc-shaped, membrane-defined cisternae arranged in a stack. Secreted proteins and glycoproteins, cell membrane proteins and glycoproteins, lysosomal proteins, and some glycolipids all pass through the Golgi structure during their maturation. In plant cells, much of the cell wall also passes through the Golgi.

The Golgi apparatus is structurally polarized, consisting of a “cis” face near the transitional region of the RER, an intermediate segment, and a “trans” face near the cell membrane. These faces are biochemically different, and the enzymatic content of each part is distinctly different. The cis face membrane is generally thinner than the others.

As secreted proteins move through the Golgi, many chemical changes can occur. The most important of these is the modification of carbohydrate groups. As mentioned above, many secretory proteins are glycosylated in the ER. In the Golgi, specific enzymes modify the oligosaccharide chains of glycoproteins, removing specific mannose residues and adding other sugars, such as glucose and sialic acid. These enzymes are collectively known as glycosidases and glycosyltransferases. Some secreted proteins will stop moving if their carbohydrate groups are incorrectly modified or not allowed to synthesize. In some cases the carbohydrate groups are essential for the stability or activity of the protein or for targeting the molecule to a specific destination.

In addition, there are proteins within the Golgi or secretory vesicles that cut several secretory proteins at specific amino acid positions. This is often the result of the activation of secretory proteins, an example of which is the conversion of inactive insulin into active insulin by the removal of a series of amino acids. If you go directly to this page, you’ll want to see a previous lesson about DNA, which provides background information on protein structure.

Chapter 10 Dna, Rna, And Protein Synthesis

As mentioned, a sequence of nucleotides represents the genetic information that makes us unique and the blueprint for who we are and what we are, and how we function. Part of this genetic information is devoted to the creation of proteins, which are essential for our body and are used in various ways. Proteins are made from information templates in our DNA, described below:

Nucleotides marked with an X are an example of a DNA sequence that would be used to code for a particular protein. Each DNA

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